VOLKSWAGEN’S admission on 22 September that it rigged emissions tests could throw light on why some major European countries have failed to meet air pollution targets.

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Failure to reduce levels of airborne nitrogen oxides (NOx) may mean more premature deaths and a higher disease burden (See “The death toll“).

VW cheated by fitting its cars with a “defeat device”, a component designed to recognise when an emissions test is under way and adjust the engine’s performance (See “How did Volkswagen rig emissions tests?“). VW’s CEO resigned over the scandal which is said to have affected 11 million cars, including VW, Audi, Skoda and Seat models.

An investigation by New Scientist suggests that no other manufacturer of diesel vehicles has gone as far as VW to pass the tests. But there may be many that legally deploy unrealistic conditions during testing to reduce strain on the engine and with it the emissions produced.

The aim is to find a performance sweet spot that maximises the apparent fuel efficiency of the vehicle and minimises its emissions, even if both deviate from how the car performs in the real world.

Strategies to reduce strain on the engine could include increasing the tyre pressure or turning off the air conditioning and heating.

1001 tricks

“None are as extreme as what VW did, but there are 1001 tricks you can play to reduce emissions during the test,” says Alastair Lewis of the University of York, UK. “The key is to accelerate as slowly and smoothly as possible to put the minimum load on the engine.”

Martin Adams, head of the air pollution group at the European Environment Agency (EEA), says it is already well known that on-road fuel consumption and emissions can be significantly higher than official test measurements indicate. “The amount of fuel cars use on the road can be 30 per cent or more higher than official measurements. For NOx, the differences are even greater, particularly for diesel vehicles, with real-life measurements showing that emissions can be as much as four or five times higher than in the test.”

“There have been rumours that some vehicles ‘know’ they are being tested,” Adams says, “but this is the first time it has been confirmed.”

If this practice is widespread, it could help explain why NOx emissions in many European countries continue to overshoot targets (see map). Transport accounts for 46 per cent of all NOx emissions in the region, according to the EEA.

EEA figures show that European Union member states have struggled to meet NOx limits set in European legislation in 2008 on the basis of what manufacturers said they could achieve technologically. By 2010, 11 of the 27 countries then in the union had failed to reach the target, and six still hadn’t by 2013, including France and Germany, the home of VW. The limit is 40 micrograms per cubic metre of air.

“In 2013, Germany and France reported the highest exceedances of NOx limits, with 218 kilotonnes and 180 kilotonnes respectively,” the EEA reported in June. The two countries were also the major overshooters in 2010. Austria, Belgium, Ireland and Luxembourg have also been persistent offenders.

We don’t know whether manufacturers’ strategies have contributed to the unexpected failure of countries to reach NOx targets, Adams says. It is one possibility, but there are other explanations.

One is the growth in overall traffic, and another the increase in cars that burn diesel instead of petrol. Diesel cars release less carbon dioxide than petrol cars, but they currently account for 80 per cent of all vehicle NOx emissions, according to EEA estimates.

New, more realistic tests have been developed and agreed, Adams says. “The sooner they are in operation, the better. It’s now down to timing. It could be as soon as 2017, or as late as 2021.”

“New emissions tests that reflect real driving conditions could be rolled out as soon as 2017”

The death toll

We know that nitrogen oxides (NOx), especially the nitrogen dioxide (NO2) discharged from car exhausts, are indirectly harmful. They contribute to the production of ozone and fine particulate matter, both of which harm human health.

According to estimates by the European Environment Agency, in 2011 ozone was the cause of about 16,000 premature deaths in the European Union, and particulate matter could be attributed to around 430,000 deaths.

But it has been difficult to establish whether NO2 itself is harmful. That is changing, says Martin Williams of King’s College London. “Now, there is much stronger evidence for independent effects of NO2.”

An investigation by the World Health Organization in 2013 found that NO2 is a hazard to health in its own right. The gas aggravates heart and lung disease, says Martin Adams, head of the air pollution group at the EEA. A UK government document published last month estimated that up to 23,500 deaths in the UK can be attributed to NO2 each year. The Europe-wide impact of NO2 hasn’t yet been established, Adams says.

The switch to diesel vehicles is making things worse: their engines produce more NOx than those that run on petrol. What’s more, around 70 per cent is in the form of NO2, compared with only 10 to 15 per cent when petrol is burned. NOx levels in many European cities are higher than the EU limit of 40 micrograms per cubic metre of air (see map).FIG-mg30414701.jpg

Proposed solutions include a ban on diesel vehicles in cities or the creation of clean air zones, fuel duty to discourage diesel use, and monetary incentives for switching to electric or hybrid vehicles.

This article appeared in print under the headline “The curious case of NOx pollution”